Man's physical universe

xanabras

252 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME

At the equator these waters move westward in the Atlantic and Pacific

oceans. The westward current in the Atlantic Ocean at the equator

divides, one branch flowing northward past the West Indies, through

the Caribbean Sea and the Gulf of Mexico, and past Florida as the

Gulf Stream. The equatorial stream in the Pacific divides near the

islands of Australia, the northern branch becoming the Japan Current.

It must not be thought, however, that the only cause of ocean currents

is the wind. It is probable that differences in the densities of water due

to differences in salt content and other factors also cause ocean currents.

These ocean currents seldom average more than three or four miles

per hour, but they tend to equalize the temperature of the globe by

making the waters of the equatorial zones cooler and the waters of

the polar regions warmer. These ocean currents, influencing the temperature

of the winds that blow over them, make the climate more

equable on the portions of the continents in line with these winds.

The Japan Current divides on reaching the western coast of North

America, the northward portion becoming the Alaska Current, and

the southward portion becoming the California Current.

Arid Regions Are Found on the Leeward Side of Well-watered Mountain

Ranges.

The mountains along the Pacific coast in the United States produce

conditions leading to the precipitation of much of the water from the

winds that sweep up and over them ; and inasmuch as these mountains

are transverse to the prevailing winds, the regions on the other side of

the mountains receive very little rain. The Appalachians present no

such effective barrier — they are not so high and are more or less

parallel to the prevailing winds during much of the year.

Climates Differ with Altitude.

The temperature of the free air decreases on the average of 1° F.

(though the rate of decrease varies widely from place to place and from

time to time) for every three hundred feet of rise, so that the climate of

the highest mountains resembles that of polar regions. The snow line

represents the altitude at which snow is found most of the year. The

snow line is higher with higher mountains and varies with the latitude,

of course.

The Inclination of the Earth's Axis to

about the Sun Is the Cause of the Seasons.

the Plane of the Earth's Orbit

Figure 77 is based on observations that the axis of the earth is

always inclined at an angle of 23 J^ degrees to the plane of the earth's

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